Method of making and using composition and energy storage device

ABSTRACT

A method is provided that includes forming brass from zinc powder and copper powder in the presence of a sodium electrolyte. In one aspect, an electrochemical cell is loaded with copper and zinc, the electrochemical cell is heated, and at least one charge/discharge cycle of the electrochemical cell is performed. Alpha brass is converted to gamma brass in presence of zinc and sodium. Methods of producing and operating an energy storage device are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority and benefit of U.S. ProvisionalApplication No. 61/095,380 entitled “COMPOSITION, ENERGY STORAGE DEVICEAND METHOD” filed on Sep. 9, 2008, which is incorporated herein byreference in its entirety.

BACKGROUND

1. Technical Field

The invention includes embodiments that relate to a method of making andusing a composition for use as a cathode material in an energy storagedevice. The invention includes embodiments that relate to methods ofmaking and using the energy storage device.

2. Discussion of Art

Work has been done on rechargeable batteries using sodium as thenegative electrode in a battery cell. Sodium has a standard reductionpotential of −2.71 volts. Sodium is relatively low weight, non-toxic,abundant, and economically desirable in the form of sodium chloride. Thesodium anode may be used in liquid form, and the melting point of sodiumis 98 degrees Celsius. An ion conducting solid electrolyte (separator)separates the liquid sodium anode from a positive electrode (cathode).

A second, molten electrolyte transports ions to and from the separatoron the cathode side. The melting point of the molten electrolyte, alongwith the temperature-dependent, sodium-ion conductivity of the solidelectrolyte, determines the minimum operating temperature of thebattery. The cathode should include a material that is soluble in themolten electrolyte and is compatibility with the solid electrolyte inthe charged (oxidized) state. Low solubility of the oxidized cathodematerial in the molten electrolyte can lead to passivation of theremaining uncharged (reduced) electrode surface and fouling of thepores.

It may be desirable to have an electrochemical cell that has differentchemistry than those electrochemical cells that are currently available.It may be desirable to have an energy storage method that differs fromthose methods that are currently available.

BRIEF DESCRIPTION

In accordance with an embodiment of the invention, a method is providedthat includes forming brass from zinc powder and copper powder in thepresence of a sodium electrolyte. In one aspect, an electrochemical cellis loaded with copper and zinc, the electrochemical cell is heated, andat least one charge/discharge cycle of the electrochemical cell isperformed.

In one embodiment, a method of converting alpha brass to gamma brass isprovided. The alpha brass is converted to gamma brass in the presence ofzinc and sodium electrolyte, or salts of zinc and sodium electrolyte.

Embodiments include a method of determining a state of charge in anenergy storage device. The determination may be made by associatingZnCl₂ concentration in the energy storage device with open circuitvoltage; associating ZnCl₂ concentration voltage with state of charge;measuring the open circuit voltage; and determining the state of chargebased on the open circuit voltage measurement, and a corresponding ZnCl₂concentration for the measured open circuit voltage, and the state ofcharge for the corresponding ZnCl₂ concentration.

In accordance with an embodiment, a method of operating an energystorage device is provided. The energy storage device has an electrodeincluding both an electrochemically active brass support structure andunalloyed zinc. The electrochemically active brass support structureincludes zinc alloyed with copper. The device is operated at anelectrical potential that is greater than an oxidation potential ofcopper during a period where the brass support structure is galvanicallyprotected from degradation.

In one embodiment, a method of forming an energy storage device isprovided that includes disposing a separator into a housing to define acathode chamber and a sealed anode chamber. The method further includesadding sodium chloride, zinc and copper into the cathode chamber andsealing of the cathode chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing potentials at differing depths of discharge.

FIG. 2 is a graph of a series of cycles showing electrolyte areaspecific resistances for a material according to an embodiment of theinvention.

FIG. 3 is a graph of charge and discharge cycle for a material accordingto an embodiment of the invention.

FIG. 4 is a graph showing potentials at differing depths of discharge.

FIG. 5 is a graph of a series of cycles showing electrolyte areaspecific resistances for a material according to an embodiment of theinvention.

FIG. 6 is a graph showing cell resistance during charge and dischargeaccording to an embodiment of the invention

DETAILED DESCRIPTION

The invention includes embodiments that relate to a method of making andusing a composition for use as a cathodic material in an electrochemicalcell in an energy storage device. The invention includes embodimentsthat relate to associated methods of making and using the energy storagedevice.

As used herein, a cathode supplies or receives electrons duringcharge/discharge of a battery. The cathode includes cathodic materialshaving differing functions: an electrode material and a supportstructure. The electrode material is present in the cathode as aparticipating electrochemical reactant both either in its oxidized orreduced state, or at some state between full oxidation or reduction. Thesupport structure does not undergo much if any chemical reaction duringthe charge/discharge, but does support the electrode material as theelectrode material undergoes chemical reaction and allows for a surfaceupon which solids may precipitate as needed. An electrolyte is a mediumthat provides the ion transport mechanism between the positive andnegative electrodes of a cell, and may act as a solvent for the oxidizedform of the electrode material. Additives that facilitate the iontransport mechanism, but do not themselves provide the mechanism, aredistinguished from the electrolyte itself.

In accordance with an embodiment of the invention, an electrode may havean electrochemically active brass support structure and unalloyed zinc.The zinc functions as a working, active electrode material in theelectrochemical cell. The electrochemically active brass supportstructure may include zinc alloyed with copper. Zinc and brass mayaccount for about 15 v/o of the cathode at 100 percent depth ofdischarge (DoD), and brass may account for about 9 v/o at 0 percent DoD,in one embodiment. In one embodiment, the electrode is a cathode.

Brass is commercially available in a wide variety of compositionsincluding zinc and copper. Small amounts of other metals, such asaluminum or tin, may be present in differing degrees based on the typeand purity of brass obtained. One suitable brass material may include amilled (−325 mesh) brass powder that has from about 27 weight percent toabout 29 weight percent zinc, from about 0.7 weight percent to about 0.9weight percent aluminum, and the remainder copper. In one embodiment,substantially all of the zinc in the cathode is alloyed with the copperto form brass. In one embodiment, the cathode includes copper and zinc,and has less than 1 weight percent of aluminum, tin, or aluminum andtin. In one embodiment, the amount of aluminum or tin, or the combinedamount of aluminum and tin, is in a range of from about 0.01 percent toabout 0.1 percent based on the weight of the combination of copper andzinc. In one embodiment, the amount of aluminum, tin, or both is zero.

In accordance with an embodiment of the invention, an energy storagedevice or an electrochemical cell is provided. The device includes ahousing having an interior surface defining a volume. A separator isdisposed in the volume. The separator has a first surface that definesat least a portion of a cathode chamber, and a second surface thatdefines an anode chamber, and the cathode chamber is in ioniccommunication with the anode chamber through the separator. In oneembodiment, the anode chamber is sealed.

The housing can be sized and shaped to have a cross-sectional profilethat is square, polygonal, or circular; and can have a width to lengthratio that is greater than about 1:10. In one embodiment, the length towidth ratio is in a range of from about 1:10 to about 1:5, from about1:5 to about 1:1, from about 1:1 to about 5:1, from about 5:1 to about10:1, from about 10:1 to about 15:1. The housing can be formed from amaterial that is a metal, ceramic, or a composite; the metal can beselected from nickel or steel, the ceramic can be a metal oxide.

The cathode chamber may include an electrolyte and the cathodicmaterial. The electrolyte and the cathodic material may be disposedwithin the cathode chamber before sealing the cathode chamber to theseparator. The cathodic material may include the support structure andan electrode material. In one embodiment, the electrode material may bezinc. In one embodiment, the electrode material may include zinc andcopper. At operating temperature the cathode chamber may contain a meltof the electrolyte and the electrode material.

The electrode material can be secured to an outer surface of the supportstructure. The support structure can have a high surface area. Theelectrode material on the support structure may be adjacent to the firstsurface of the separator and extend away from that separator surface.The support structure can extend away from the first surface to athickness that is greater than about 0.01 millimeters. In oneembodiment, the thickness is in a range of from about 0.01 millimetersto about 0.1 millimeter, from about 0.1 millimeters to about 1millimeter, from about 1 millimeter to about 5 millimeters, from about 5millimeters to about 10 millimeters, from about 10 millimeters to about15 millimeters, from about 15 millimeters to about 20 millimeters. Forlarger capacity electrochemical cell, the thickness may be larger than20 millimeters.

Anchoring the electrode material to a support structure surface, ratherthan filling the cathode chamber with only the melt, can control moltensalt homogeneity. That is, the placement of the electrode material onthe support structure allows for the ability to locate specificmaterials within the electrochemical cell. In one example, pooling ofmolten electrolyte compounds at the bottom of the cathodic chamber canbe avoided. Or, for example, a concentration of a first electrodematerial in elemental form may differ from a point nearer the separatorrelative to a point further from the separator. Like an onion, there canbe layers of the electrode material present in different concentrationsor amounts depending on where located inside the electrochemical cell.Similarly, a gradient can be formed to address, for example, electricalresistivity rise or to provide a more constant availability of reactantas the reaction front area changes as it is used and advances into thecell body away from the separator surface. A gradient, as used herein,can include a concentration step change, and as such can be configuredto function as a state of charge indicator.

The support structure, in an electrochemical reaction, may not beoxidized by the electrochemical reaction at a determined maximum cellpotential that is less than the copper oxidation potential but greaterthan the zinc oxidation potential. As noted, however, due to thegalvanic protection of the copper by the zinc, the brass supportstructure is galavanically protected from degradation and also it may bepossible to run the cell at an electrical potential higher than theoxidation potential of the copper and still not oxidize the copper.Further, if the zinc is present in sufficient quantities the zinc maymaintain the copper in a non-ionic form or a metallic form. In otherwords, the copper may be precluded from forming the ionic species, orthe zinc may reduce or prevent migration of copper into the separator.

Copper itself has an oxidation potential of 2.6 volts versus sodium inthe present case, where the electrolyte is 400 degrees Celsius sodiumtetrachloroaluminate saturated with sodium chloride. Copper ions aresignificantly soluble in the molten salt, and if not addressed canexchange into a sodium beta″-alumina separator, which can lead to adecrease in conductivity and to catastrophic fracture. Provided there iszinc left in the cell, either as metallic zinc or as part of the brassalloy, zinc reduces mobile copper ions back to metallic copper andreduces or eliminates copper migration into the separator via ionexchange. The zinc may act as a sacrificial anode relative to thecopper. This may enable cell operation up to voltages above the copperoxidation potential, without the expected oxidation of copper.

At least one of the zinc or the copper may be in the form of powder,fiber, foam, or foil. Naturally, the smaller the particles the moreactive surface area is available for reaction. The initial form of thereactants may not be retained whence charge/discharge cycling iscomplete. That is, a cell packed initially with powder may change toporous foam after the first use. The foam may be an open cell orreticulated type.

The pores may be present as mono-modal or as multi-modal average poresize distributions. In a mono-modal distribution, one mode may have anaverage pore size of less than about 150 micrometers, and in oneembodiment the average pore size is about 100 micrometers. In a bi-modaldistribution, the first mode may have an average pore size in a range offrom about 90 micrometers to about 125 micrometers; and, the second modemay have an average pore size in a range of from about 5 micrometers toabout 20 micrometers. For example, a suitable bi-modal distribution mayinclude 100 micrometer pores and 10 micrometer pores. The 10 micrometerpores may be formed from electrochemical etching after the initial poresare formed. Such electrochemically etched pores may constitute about 10percent of the total pore volume. Pore size is measured as the widestspacing along a length of a porous opening.

A cell that includes the cathodic material has an initial state prior toits first use or charge/discharge, and a subsequent state thatrepresents the cathodic material after the first use orcharge/discharge. The distinction is notable for several reasons. Onereason is that the cell can be loaded with unalloyed zinc and copper,which then form a brass alloy during the first use or charge/discharge.Or, the cell can be loaded with brass initially, or can be loaded withsome combination of zinc, copper and brass. As noted elsewhere herein,the initial loading can be in the form of powder, granules/grains,shavings, wire, foil or other physical configuration. In the subsequentstate, the brass is generally a porous foam regardless of the physicalconfiguration in the initial state. Granulation of brass particles addedto the cell can influence or control the foam properties, and poreformers can be added to further control the foam porosity and poreconfigurations.

Further, the distribution of ingredients needs not to be homogeneous ineither of the initial or the subsequent state. A separator and currentcollector, discussed further hereinbelow, make reasonable referencepoints for a discussion of metal concentrations. In one embodiment, inthe subsequent state, the cathodic material is zinc rich proximate theseparator and is copper rich proximate the current collector. The ratioof zinc to copper can change over the distance from the separator to thecurrent collector in a manner that is linear or non-linear. In anon-linear instance, the ratio change is semi-exponential. Duringoperation then, the reaction of zinc at the first location can occurprior to reacting the zinc at the second location during the at leastone charge/discharge cycle.

As noted above, the brass can be added to the cell in the initial state.That brass can be alpha brass. And, the zinc can be disposed in theenergy storage device in a manner that facilitates participation of thezinc in a chemical reaction with the alpha brass to form the gamma brassin the subsequent state. That is, the alpha brass may be converted togamma brass in presence of zinc and sodium electrolyte during the atleast one charge/discharge cycle. The gamma brass can form a layer onthe support structure with alpha brass encased therein. The supportstructure can thus include only alpha brass in the initial state, andcan include both alpha brass and gamma brass in the subsequent state. Inone embodiment, in the initial state the alpha brass is free ofaluminum, tin, or both aluminum and tin.

With regard to the brass support structure, the brass includes a layerof gamma brass at a surface of the support structure. The layer ofgamma-brass may have a thickness that is less than about 10 micrometer.In one embodiment, the thickness of the layer is in a range of fromabout 1 micrometer to about 2 micrometer, from about 2 micrometer toabout 4 micrometer, from about 4 micrometer to about 6 micrometer, fromabout 6 micrometer to about 8 micrometer, from about 8 micrometer toabout 10 micrometer. In one embodiment, the thickness of the layer isless than about 1 micrometer.

The ionic material transported across the separator between the anodechamber and the cathode chamber can be an alkali metal. Suitable ionicmaterial may include one or more of sodium, lithium and potassium. Theanodic material is molten during use. The anode chamber may receive andstore a reservoir of anodic material. Additives suitable for use in theanodic material may include a metal oxygen scavenger. Suitable metaloxygen scavengers may include one or more of manganese, vanadium,zirconium, aluminum, or titanium. Other useful additives may includematerials that increase wetting of the separator surface by the moltenanodic material. Additionally, some additives may enhance the contact orwetting between the separator and the current collector, to ensuresubstantially uniform current flow throughout the separator.

Other additives may affect performance. Such performance additives mayincrease ionic conductivity, increase or decrease solubility of thecharged cathodic species, reduce vapor pressure over the electrode,improve wetting of the solid electrolyte by the molten electrolyte, orprevent ripening of the cathode microdomains, to name several utilities.In one embodiment, the additive may be present in an amount that is lessthan about 5 mole percent compared to the total combined moles of alkalimetal halide, aluminum halide and zinc halide.

The presence of a sulfur- or phosphorous-containing additive may bedisposed in the cathodic material. The presence of sulfur or phosphorousin the cathode prevents recrystallization of salts and grain growth. Forexample, elemental sulfur, sodium sulfide, or triphenyl sulfide may bedisposed in the cathode.

A suitable electrolyte may include a binary electrolyte or a ternaryelectrolyte. In a ternary electrolyte, alkali metal halide and zinchalide may be present in a determined amount relative to aluminumhalide. The amount of the zinc halide present in the ternary electrolytemay be greater than about 20 mole percent relative to the amount of thealuminum halide.

In addition to the alkali metal halide and the zinc halide, the balanceof the ternary electrolyte is aluminum halide. With regard to the halideportion, each of the halides of the ternary electrolyte may have acommon halide, such as chloride. Alternately, the common halide mayinclude bromide, iodide or fluoride. In one embodiment, the halide mayinclude chloride and one or more additional halides. If a second oradditional halide is present, then relative to the common halide theamount of additional halide may be greater than about 0.1 mole percentof the total halide concentration. In one embodiment, the additionalhalide amount is in range of from about 0.1 mole percent to about 0.5mole percent, from about 0.5 mole percent to about 1 mole percent, fromabout 1 mole percent to about 5 mole percent, or from about 5 molepercent to about 10 mole percent.

The electrolyte has a nominal average melting point of about 150 degreeCelsius. Suitable operating temperature for the electrochemical cellhaving the electrolyte may be greater than about 350 degrees Celsius. Atlower operating temperature, below 350 degrees Celsius, the cell(ternary electrolyte) may have relatively higher resistance. Theelectrolyte may have an ionic resistivity that is greater than about 1.0ohm-centimeter. In one embodiment, the electrolyte may have ionicresistivity that is in a range of from about 1.0 ohm-centimeter to about1.5 ohm-centimeter, from about 1.5 ohm-centimeter to about 1.6ohm-centimeter, from about 1.6 ohm-centimeter to about 1.7ohm-centimeter, from about 1.7 ohm-centimeter to about 1.8ohm-centimeter, from about 1.8 ohm-centimeter to about 1.9ohm-centimeter, from about 1.9 ohm-centimeter to about 2.0ohm-centimeter, or from about 2.0 ohm-centimeter to about 2.5ohm-centimeter. Ions of interest include alkali metal ions.

The electrolyte may be disposed within the cathode chamber defined bythe separator. The electrolyte may not infiltrate zinc ions from thezinc halide into the separator during use, where such infiltration isundesirable. That is, the electrolyte may be compatible with thecathodic material and the separator and thus may allow the rapidtransport of ions from and to the cathodic material to and from theseparator. The electrolyte may provide less ionic resistivity for alkalimetal ions. Thus, use of the electrolyte may provide high power and highefficiency electrochemical cell operation.

As noted above, the separator is disposed within the volume of thehousing. The separator may have a cross-sectional profile normal to theaxis that is a circle, a triangle, a square, a cross, or a star.Alternatively, the separator can be about planar. A planar configuration(or with a slight dome) may be useful in a prismatic or button-typebattery configuration, where the separator is domed or dimpled.Similarly, the separator can be flat or undulate.

The separator is an alkali metal ion conductor solid electrolyte thatconducts alkali metal ions during use. Suitable materials for theseparators may include an alkali-metal-beta′-alumina,alkali-metal-beta″-alumina, alkali-metal-beta′-gallate, oralkali-metal-beta″-gallate. In one embodiment, the separator includes abeta alumina. In one embodiment, a portion of the separator is alphaalumina and another portion of the separator is beta alumina. The alphaalumina may be relatively more amenable to bonding (e.g., compressionbonding) than beta alumina, and may help with sealing and/or fabricationof the energy storage device.

The separator may be stabilized by the addition of small amounts of, butnot limited to lithia, magnesia, zinc oxide, yttria or similar oxides.These stabilizers may be used alone or in combination with themselves orwith other materials. The separator, sometimes referred to as betaalumina separator electrolyte (BASE) may include one or more dopant.Suitable dopants may include oxide of a transition metal selected fromiron, nickel, copper, chromium, manganese, cobalt, or molybdenum. Aseparator having the dopants is referred to as beta″ alumina separatorelectrolyte, and has higher sodium ion conductivity than beta alumina.Sodium ion conductivity of one form of beta″ alumina separatorelectrolyte at 300 degrees Celsius is in a range of from about 0.2 ohm⁻¹cm⁻¹ to about 0.4 ohm⁻¹ cm⁻¹.

The amount of the stabilizer to the beta″ alumina can be greater than0.5 weight percent. In one embodiment, the amount is in a range of fromabout 0.5 weight percent to about 1 weight percent, from about 1 weightpercent to about 2 weight percent, from about 2 weight percent to about3 weight percent, from about 3 weight percent to about 4 weight percent,from about 4 weight percent to about 5 weight percent, from about 5weight percent to about 10 weight percent, from about 10 weight percentto about 15 weight percent, from about 15 weight percent to about 20weight percent, or greater than about 20 weight percent based on thetotal weight of the beta″ alumina material.

The separator can be a tubular container in one embodiment having atleast one wall. The wall can have a thickness; and an ionic conductivityand the resistance across the wall may depend in part on the thickness.Suitable thickness can be less than 5 millimeters. In one embodiment,the thickness is in a range of from about 5 millimeters to about 4millimeters, from about 4 millimeters to about 3 millimeters, from about3 millimeters to about 2 millimeters, from about 2 millimeters to about1.5 millimeters, from about 1.5 millimeters to about 1.25 millimeters,from about 1.25 millimeters to about 1.1 millimeters, from about 1.1millimeters to about 1 millimeter, from about 1 millimeter to about 0.75millimeters, from about 0.75 millimeters to about 0.6 millimeters, fromabout 0.6 millimeters to about 0.5 millimeters, from about 0.5millimeters to about 0.4 millimeters, from about 0.4 millimeters toabout 0.3 millimeters, or less than about 0.3 millimeters.

A cation facilitator material can be disposed on at least one surface ofthe separator, in one embodiment. The cation facilitator material mayinclude, for example, selenium. At least one separator surface has asurface roughness RMS in a range of greater than about 10 nanometers. Inone embodiment, the surface roughness (RMS) is in range of from about 10nanometers to about 20 nanometers, from about 20 nanometers to about 30nanometers, from about 30 nanometers to about 40 nanometers, from about40 nanometers to about 50 nanometers, from about 50 nanometers to about60 nanometers, from about 60 nanometers to about 70 nanometers, fromabout 70 nanometers to about 80 nanometers, from about 80 nanometers toabout 90 nanometers, from about 90 nanometers to about 100 nanometers.

Optionally, one or more shim structures can be disposed with the volume.The shim structures support the separator within the volume of thehousing. The shim structures can protect the separator from vibrationscaused by motion of the cell during use and thus reduce or eliminatemovement of the separator relative to the housing. If present, the shimstructures may function as a current collector to the housing.Utilization of the shim structures as a current collector may be usefulif the molten anode level rises and falls during charge and discharge.The shim structures may provide a thin gap adjacent to the separator tofacilitate wicking of a thin layer of molten anodic material against asurface of the separator. This wicking may be independent of the stateof charge of the battery, and independent of the head height of anodicmaterial.

The separator can seal to an inner surface of the housing by a sealingstructure. The sealing structure can be of a glassy composition. Thesealing structure is operable to maintain a seal between the contentsand the environment at a temperature greater than about 100 degreesCelsius. In one embodiment, the operating temperature range is fromabout 100 degrees Celsius to about 200 degrees Celsius, from about 200degrees Celsius to about 300 degrees Celsius, from about 300 degreesCelsius to 400 degrees Celsius, from about 400 degrees Celsius to about500 degrees Celsius, from about 500 degrees Celsius to about 600 degreesCelsius.

Suitable glassy sealing compositions may include, but are not limited tophosphates, silicates, borates, germinates, vanadates, zirconates,arsenates, and their various forms, for example, borosilicates, aluminosilicate, calcium silicate, binary alkali silicates, alkali borates, ora combination of two or more thereof. Alternatively, the end portions ofthe separator may include alpha alumina. The alpha alumina can be bondeddirectly to the lid that encloses the cathode chamber. Suitable bondingmethods may include thermal compression bonding, diffusion bonding, orthin film metallizing, each of these may be used in conjunction withwelding or brazing techniques.

The separator can be formed as a toughened ceramic, and can be formedwith various modifiers that affect physical strength, vibration/shockresistance, ionic conductivity/resistance, and copper ion infiltration.To reduce a pressure differential across the separator, the negativepressure generally caused on the cathode side by the migration of ionsto the anode may be balanced by reducing the initial pressure on theanode side to less than ambient. The anode side may be sealed undervacuum, or a vacuum may be formed after sealing by reacting a consumablegas (oxygen) in the anode chamber with the incoming ions (e.g., sodiumto solid sodium oxide) during initial usage.

The energy storage device may have a plurality of current collectorsincluding anode current collectors and cathode current collectors. Theanode current collector is in electrical communication with the anodechamber and the cathode current collector is in electrical communicationwith the contents of the cathode chamber. Suitable materials for theanode current collector may include W, Ti, Ni, Cu, Mo or combinations oftwo or more thereof. Other suitable materials for the anode currentcollector may include carbon. The cathode current collector may be awire, paddle or mesh formed from Pt, Pd, Au, Ni, Cu, C, or Ti. Thecurrent collector may be plated or clad. In one embodiment, the currentcollector is free of iron.

The plurality of current collectors can have thickness greater than 1millimeter (mm). In one embodiment, the thickness is in a range of fromabout 1 millimeter to about 10 millimeters, from about 10 millimeters toabout 20 millimeters, from about 20 millimeters to about 30 millimeters,from about 30 millimeters to about 40 millimeters, or from about 40millimeters to about to about 50 millimeters. Cladding on the currentcollector, if present, may coat the current collector to a thicknessgreater than about 1 μm. In one embodiment, the cladding thickness is ina range of from about 1 micrometer (μm) to about 10 μm, from about 10 μmto about 20 μm, from about 20 μm to about 30 μm, from about 30 μm toabout 40 μm, or from about 40 μm to about to about 50 μm.

The device may be an electrochemical cell, which be assembled in thedischarged state. Applying a voltage between the anode and the cathodeof the electrochemical cell may charge the electrochemical cell. In oneembodiment, sodium chloride in the cathode dissolves to form sodium ionsand chloride ions during charging. Sodium ions, under the influence ofapplied electrical potential, conduct through the separator and combinewith electrons from the external circuit to form the sodium electrodeand chloride ions react with the cathodic material to form metalchloride and donate electrons back to external circuit. Duringdischarge, sodium ions conduct back through the separator reversing thereaction, and generating electrons. The cell reaction is as follows:

nNaCl+M

MCl_(n)+nNa⁺+ne⁻

The electrochemical cell includes the separator having active area A;and the cell is capable of repeatedly storing and discharging a quantityof charge Q, the resistance between the two terminals is R; and througha full isothermal charge or discharge of Q, a ratio RA/Q remains in arange of from about 1.5×10⁻⁶ ohm-m²/amp-hr to about 9.2×10⁻⁶ohm-m²/amp-hr. In one embodiment, the cell can have a resistancedifferential that is less than 10 percent, where the resistance isexpressed in Ohms, on discharge, when comparing at about 0 amp-hoursrelative to about 35 amp-hours, and at a temperature that is greaterthan about 350 degrees Celsius.

A plurality of the electrochemical cells can be organized into an energystorage system. Multiple cells can be connected in series or parallel.For convenience, a group of coupled cells may be referred to as a moduleor pack. The ratings for the power and energy of the module may dependon such factors as the number of cells in the module. Other factors maybe based on end-use application specific criteria.

Various embodiments of the energy storage system can store an amount ofenergy that is in a range of from about 0.1 kilowatt hours (kWh) toabout 100 kWh. One embodiment of the energy storage system has anenergy-by-weight ratio of greater than 100 Watt-Hours/kilogram, and/oran energy-by-volume ratio of greater than 160 Watt-Hours/liter. Anotherembodiment of the energy storage system has a specific power rating ofgreater than 150 Watts/kilogram.

Suitable energy storage system may have an application specific Power toEnergy ratio of less than 10 to 1 hour⁻¹. In one embodiment, thespecific power to energy ratio is in range from about 1:1 to about 2:1,from about 2:1 to about 4:1, from about 4:1 to about 6:1, from about 6:1to about 8:1, or from about 8:1 to about 10:1. In other embodiments, thepower to energy ratio is in range from about 1:1 to about 1:2, fromabout 1:2 to about 1:4, from about 1:4 to about 1:6, from about 1:6 toabout 1:8, or from about 1:8 to about 1:10.

In one embodiment of the energy storage system, a controllercommunicates with the plurality of the cells. The controller candistribute an electrical load to select cells in a cell module inresponse to feedback signals indicating states for each of the cells inthe cell module. The controller can perform a re-warm method in which aseries of heating elements are activated in a sequence to melt a frozenportion of the energy storage device in a determined manner. In anotherembodiment, the controller may distribute an electrical load to selectcathodic materials at determined locations within individual cells.

The controller can determine a state of charge in the energy storagedevice based on the chemistry of the cell. A suitable state of chargedetermination can be made by associating ZnCl₂ concentration in theenergy storage device with an open circuit voltage; associating ZnCl₂concentration voltage with state of charge; measuring the open circuitvoltage; and determining the state of charge based on the open circuitvoltage measurement, and a corresponding ZnCl₂ concentration for themeasured open circuit voltage, and the state of charge for thecorresponding ZnCl₂ concentration. This is possible owing to therelationship between open circuit voltage and ZnCl₂ concentration—which,in turn, relates to the state of charge.

If present, a heat management device maintains the temperature of theenergy storage system. The heat management device can warm the energystorage system if too cold, and can cool the energy storage system iftoo warm. The heat management system includes a thaw profile that canmaintain a minimal heat level in the anode and cathode chambers to avoida freeze of cell reagents.

Another embodiment of the invention provides an energy management systemthat includes a second energy storage device that differs from theenergy storage device. This duel energy storage device system canaddress the ratio of power to energy in that a first energy storagedevice can be optimized for efficient energy storage, and a secondenergy storage device can be optimized for power delivery. The controlsystem can draw from either energy storage device as needed, and chargeback either energy storage device that needs such a charge.

Suitable second energy storage devices, for the power piece, include aprimary battery, a secondary battery, a fuel cell, or an ultracapacitor.A suitable secondary battery may be a lithium battery, lithium ionbattery, lithium polymer battery, or a nickel metal hydride battery.

EXAMPLES

The following examples illustrate methods and embodiments in accordancewith the invention, and as such do not limit the claims. Unlessspecified otherwise, all ingredients are commercially available fromsuch common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.),Spectrum Chemical Mfg. Corp. (Gardena, Calif.) and the like.

Example 1 Compression Molding a Cathode

The solid components of a cathodic material include zinc (Alfa AesarItem #00424, −100 mesh, 99.9% metals basis), brass (Alfa Aesar Item#43213, −325 mesh, 99.9% metals basis), and sodium chloride (Alfa AesarItem #87605, 99.99%). To increase the surface area for improved masstransfer, the sodium chloride is milled to −200 mesh in a laboratorymill in a dry glove box.

Ammonium bicarbonate [Sigma-Aldrich Item #A6141, 99.0% reagent grade) ismixed with the cathode precursor solids as a pore former. After 1 min ofmechanical milling in an inert-atmosphere glove box, the ammoniumbicarbonate powder is sieved, and the +170 mesh powder is retained foruse in the cathode preparation. The presence and amount of ammoniumbicarbonate influences cathode performance. Addition of 10-20 w/oammonium bicarbonate (13-30 v/o), results in significantly highermolding pressures to achieve a determined packing density. Beneficially,this results in moldings that have relatively higher mechanicalintegrity.

Pressing of the cathodic material powders is done in a 2 centimeterdiameter, three-part, matched-die press tool. The powders are mixed inan inert-atmosphere glove box, and then loaded into the die just priorto molding. The powders are tamped down using a brass rod to create aneven and uniform layer in a standard die. The pusher rod is markedbeforehand to a mold height of 0.5 centimeter, the desired height of themolded cathode.

The die tool is loaded into a hydraulic press. Press force is manuallyincreased until the design mold height is achieved, with maximum forcesranging from 15-100 kiloNewtons, depending on the pore-former weightfraction. After pressing, the pore former is removed from the moldeddisc in a vacuum oven at 150 degrees Celsius for one hour. Ammoniumbicarbonate decomposes to ammonia, carbon dioxide, and water at 30degrees Celsius to 60 degrees Celsius. The molded disc is cooled. Themolded disc is transferred to an inert-atmosphere glove box to minimizemoisture gain. The combination of compression molding and removal ofpore former results in a molding of the desired porosity, in this caseabout 47 percent of theoretical.

Example 2 Producing an Electrolyte

Sodium chloride and aluminum chloride are mixed and melted together toproduce sodium tetrachloro aluminate (NaAlCl₄). Aluminum chloride isvolatile when melted, so mixing and melting of the binary salt is doneas a separate step before electrochemical cell fabrication.

Preparation of the binary salt is done in a nitrogen purge box to keepthe materials as dry as possible. To produce a 750 gram batch ofNaCl-rich (basic) sodium tetrachloro aluminate, 500 g of aluminumchloride (Sigma-Aldrich Item #294713, 99.99% reagent grade) and 250 g ofsodium chloride (Sigma-Aldrich Item #204439, 99.999% reagent grade) aremixed in a 500-mL reaction vessel. The reaction vessel is sealed with aclamped lid. The lid is equipped with a gas outlet connected to amineral oil bubbler to relieve any pressure.

The reaction vessel containing the dry powders is heated to 300 degreesCelsius, which is above the melting point of the binary salt mixture.Once melted, 5-10 grams of aluminum powder (Alfa Aesar Item #42919,−100+325 mesh, 99.97% metals basis) is introduced to the molten salt.The aluminum powder, which oxidizes readily, acts to scavenge metallicimpurities present in the raw materials as cationic species.

Once melted, with impurities precipitated out, the sodium tetrachloroaluminate is filtered to remove the aluminum powder and theprecipitates. The molten salt is pipetted onto a heated (from about 200to about 300 degrees Celsius) glass frit (25-μm minimum pore size). Thefiltered molten salt is collected on aluminum foil. Once the filteredmolten salt has solidified, it is manually chipped into smaller pieces,then milled in a dedicated, laboratory-scale, grinding mill for 60seconds. The sodium tetrachloro aluminate powder is stored in a glovebox for use in cell fabrication as a binary electrolyte. Where needed, aportion of the sodium tetrachloro aluminate powder is combined with zincchloride salt and sodium chloride to produce a ternary electrolyte,which is stored in a glove box for use in cell fabrication.

Example 3 Preparation of Electrochemical Cells for Samples 1-4

Mixtures of sodium chloride, aluminum chloride, and zinc chloride areprepared as the electrolyte precursor material. A series of samples areformed in which the mole ratio, respectively, of sodium chloride,aluminum chloride and zinc chloride in the samples are 1) 4:1:0.2; 2)4:1:0.5; 3) 4:1:1; and 4) 4:1:2.

Four separator tubes, cylindrical in shape, are produced according toknown methods or are commercially obtained. Each tube is ceramic sodiumconductive beta″ alumina. The cylinder dimensions are 228 mm length, 36mm internal diameter, and 38 mm outside diameter. Each ceramic separatortube is glass sealed to an alpha alumina collar to form an assembly.Each assembly is placed in a stainless steel can that is the housing toform an electrochemical cell. The can size is about 38 mm×38 mm×230 mm.

The cathodic material includes zinc powder and alpha-brass powder. Thecathodic material is disposed in the volume of the housing, and withinthe separator tube, and functions as the working cathode. The brassfunctions as a current collector and is in electrical communication withleads allowing that to happen. The housing functions as a second currentcollector.

The temperature of each of the four formed electrochemical cells isincreased at a controlled rate (5 degrees Celsius per minute) throughthe melt temperature of the ternary electrolyte precursors, and up to anoperating temperature of 400 degrees Celsius. The ternary electrolyteforms in situ. Each current collector has external wire ends. Theexternal wire ends are connected to a computer-interfaced galvanostat(PARSTAT 2273 available from AMETEK Princeton Applied Research (OakRidge, Tenn.)) and constant current data are measured.

Results show acceptable stability of the separator, a low electricalresistance rise. That is, the cells perform desirably. The open circuitvoltage measured is variable based on the amount or concentration of thezinc chloride. A graph can be constructed that relates the measured opencircuit voltage to zinc chloride concentration, and the zinc chlorideconcentration (or activity) relates to the state of charge.

Characterization of the cathodic material after use with back-scatterelectron microscopy and wavelength dispersive X-ray analysis indicatesthat the brass alloy has undergone a partial transformation, at least onthe surface, from alpha brass to gamma brass.

Example 4 Preparation of Electrochemical Cells for Samples 5-8

Samples 5-8 are created and formed in corresponding test cells, themolar ratios of electrolyte in Samples 5-8 are correspondingly the sameas the electrolyte in Samples 1-4 as described in Example 3. Thedifference being that instead of the cathodic material being zinc andbrass, the cathodic material is a dry mix of granulated zinc metal,copper metal, and sodium chloride. A rod shaped molybdenum-based currentcollector is disposed in the ceramic separator. The dry granulated mixis dry mixed with the electrolyte mix to form four fill batches. Each ofthe four fill batches is added into a ceramic separator around thecurrent collector.

The temperature of each of the four formed electrochemical cells isincreased at a controlled rate (5 degrees Celsius per minute) throughthe melt temperature of the ternary electrolyte precursors, and up to anoperating temperature of 400 degrees Celsius. The ternary electrolyteforms in situ. Each current collector has external wire ends. Theexternal wire ends are connected to a computer-interfaced galvanostat(PARSTAT 2273 available from AMETEK Princeton Applied Research (OakRidge, Tenn.) and constant current data are measured.

Results show acceptable stability of the separator, a low electricalresistance rise. That is, the cells perform desirably. The open circuitvoltage measured is variable based on the amount or concentration of thezinc chloride. A graph can be constructed that relates the measured opencircuit voltage to zinc chloride concentration, and the zinc chlorideconcentration (or activity) relates to the state of charge.

Observation of the cathodic material after electrochemical cyclingindicates that the zinc and copper powders loaded have formed a brassalloy. The brass is zinc rich proximate the separator, and is copperrich proximate the current collector.

Example 5 Preparation of Electrochemical Cells for Samples 9-10

Before a cathode disc is loaded into the test cell, the disc is vacuumimpregnated with molten sodium tetrachloroaluminate by submerging thedisc in a heated flask of the molten salt at 300 degrees Celsius, andthen pumping a rough vacuum for one hour.

The hot disk is transferred to the separator (β″-Alumina SolidElectrolyte, sometimes referred to as BASE), a 2-cm inner diameter,1.5-mm wall thickness, 4-cm tall, flat-bottom tube (Ionotec Ltd,Cheshire, England). A quantity of additional sodium tetrachloroaluminateis added to the cathode chamber to ensure the cathode is fully envelopedwith molten-salt electrolyte. The cathode current collector, a 0.63-cmdiameter brass rod with a 1.8-cm diameter brass disc press fit onto theend of the rod, rests on top of the molded disc. The cathode chamber iscovered by a machined ceramic cap that fits around the current collectorand inside the top of the separator. The cap minimizes evaporativelosses from the molten electrolyte and minimizes crossover of sodiumvapor from the anode chamber.

The cathode/separator assembly is placed in a stainless-steel cruciblecontaining 1-2 grams of metallic sodium. The crucible serves as theanode chamber; two 2-mm stainless-steel wires spot-welded to thecrucible serve as anode current collectors.

The completed cell is heated to 400 degrees Celsius. This operatingpoint is above the melting point of Na₂ZnCl₄, but below the meltingpoint of metallic zinc, which may be about 418 degrees Celsius.

The contents of two cells built in this cell design are given inTable 1. Sample 9 is designed to utilize zinc present in the brasssupport material during charging, while Sample 10 utilizes only metalliczinc added to the brass support. The electrolyte surface areas in Table1 correspond to the area in contact with the molded cathode. The wettedanode area is larger.

TABLE 1 Ingredient list for Samples 9-10. Sample 9 Sample 10 Brass 2.32g 1.42 g Zinc 0.28 g 0.63 g Sodium Chloride 1.12 g 1.68 g AmmoniumBicarbonate 0.41 g 0.41 g (before baking) Sodium tetrachloro 2.51 g 2.51g aluminate Sodium 2 g 2 g Separator surface area 6.28 cm² 5.97 cm²

Testing of the Sample 9-10 cells is performed on a 10-amp, 10-V,4-channel, Princeton Applied Research (PAR) VMP3 model potentiostat.Charging and discharging of the cells is performed at 0.01 amp/cm² to0.3 amp/cm² of separator (ceramic electrolyte) surface area. Cells arecycled under constant-current control with limits on maximum (charging)and minimum (discharging) cell potential, followed by furtherelectrolysis at constant cell potential. Each half-cycle is ended whenthe design charge capacity is reached. The temperature at the outside ofthe anode is 400 degrees Celsius.

Sample 9 illustrates the participation of metallic and brass-bound zincin oxidation. Cycling protocol includes the following. Four charge anddischarge cycles are performed for Sample 9. In these cycles, the cellis charged at 0.06 amp under constant-current control to a voltage of2.6 V versus sodium. At 2.6 V, the cell is held at constant voltageuntil the design charge capacity of 0.5 amp-hr is reached. The cell isdischarged at negative 0.06 amps under constant-current control to avoltage of 1.7 V. At 1.7 V, the cell is held at constant voltage untilthe designed capacity is reached. During charging and discharging, theconstant-current steps are periodically paused for 300 seconds tomeasure the open-circuit potential of the cell.

Open-circuit potential, measured in the rest periods, is shown inFIG. 1. U₀ is seen to rise with decreasing DoD. The U₀ curves aresomewhat different during charging, U_(0C), and discharging, U_(0D),with the discharging curve at slightly lower potential. Thesedifferences are attributed to differences in electrode surfacecomposition during charging (Cu-rich) and discharging (Zn-rich), as wellas to lack of complete equilibration in the electrolyte during the 300second rest period.

Enough metallic zinc is included in the initial composition to support0.23 amp-hr of charging (54% DoD). Below 54% DoD, brassbound zinc mustparticipate in oxidation, assuming that all metallic zinc is oxidizedfirst. Charging is represented by a line indicated by the referencenumber 10, and discharge by a line indicated by reference number 20.Brassbound zinc may oxidize at a potential that is intermediate betweenthose potentials for zinc and copper. Brassbound zinc may preferentiallyoxidize before the copper, which may keep the copper from becoming amobile ionic species that may threaten the integrity of the separator.The standard reduction potential of Cu⁺⁺ is 0.6 V higher than that forZn⁺⁺. During discharge, zinc precipitates onto the brass support, andthe electrode surface is expected to be richer in zinc, than at the sameDoD during charging.

With regard to charging and discharging resistances, current interruptsare used to calculate the resistance of the cell. The resistance of thecell is calculated by:

${Rj} = \frac{U - U_{0j\; 15}}{I}$

where U and I are the charging potential and current, measuredimmediately prior to an interrupt, and U_(0j15) is the potentialmeasured 15 seconds after the current interrupt. 15 seconds is longenough to capture purely ohmic resistance and short enough, so thatre-equilibration processes involving long-range mass transport are notincluded in the calculation. j=C for charging; and, =D for discharging.

FIG. 2 shows the electrolyte-area-specific resistances, ρ^(C) and ρ^(D),for Sample 9 (cycles one through five). For each cycle dataset, the leftside of the panel corresponds to 100% DoD, and the right side to 0% DoD.

In each cycle, the discharging resistance (30) is higher than thecharging resistance (40). The solids volume fraction in the cathodeincreases with DoD, due possibly to the deposition of NaCl. Bothcharging and discharging in the cathode are expected to initiate at theβ″-alumina interface and move inward as electrolysis proceeds. In thecase of charging, porosity is high in the wake of the reaction front,and the effective ionic conductivity is high. In the case ofdischarging, porosity is low in the wake of the reaction front, as isthe effective ionic conductivity.

Also of note is the increase in charging resistance, cycle over cycle,increasing nearly 300% in only five cycles. While the trend indischarging resistance is less clear, the resistance by cycle five ishigher than in cycle one (roughly 200% increase). At 0% DoD, 97% of theNaCl has been consumed by the oxidation reaction. In this state, themolten salt composition is acidic in the sense that the Group 12 and 13metal chlorides, ZnCl₂ and AlCl₃, chloride-accepting Lewis acids, arenot balanced by a full complement of alkali NaCl, a chloride-donor Lewisbase: one NaCl to each AlCl₃ and two NaCl to each ZnCl₂. Under acidicconditions, there may be reduced conductivity in the β″-alumina. In theabsence of balancing NaCl, the vapor pressure over the moltenelectrolyte may increase, which can lead to evaporation of theelectrolyte. Both of these phenomena may result in increased cellresistance.

One charging/discharging cycle of Sample 10 is shown in FIG. 3. As shownby the charge curve (50), the capacity of Sample 10 is equivalent tothat of Sample 9, which is 0.5 amp-hr. In this cycle, the Sample 10 cellis charged at 0.165 amp in current control (60) until the design chargecapacity of 0.5 Ah is reached. After a one hour rest, the Sample 10 cellis then discharged at negative 0.165 amp in current control (70), untilthe design capacity is reached. In both charge and discharge, theconstant current steps are periodically interrupted for 300 seconds tomeasure the open-circuit potential, U_(0C) and U_(0D), of the cell.

FIG. 5 shows ρ^(C) and ρ^(D) for cycles one, four, seven, eight andnine, for Sample 10, similar to FIG. 2. In each panel, DoD varies from100% to 0% going left to right. In the case of Sample 10, little changeis seen from cycle to cycle. In most cycles the discharge resistance(100) is higher than the charge resistance (110), although the two aremuch closer than in Sample 9.

U_(0C) (80) and U_(0D) (90) measured during the periodic rests are shownin FIG. 4. Compared to Sample 9, the potentials at 0% depth of discharge(DoD) and 100% DoD in Sample 10 are lower. From FIG. 4, <U_(0D)>Q (stateof charge average of U_(0D)) is calculated to be 1.941 V over 0.5 amp-hrof discharge. The maximum discharge energy, is WDmax=(1.941 V)(0.5amp-hr)=0.97 W-hr for Sample 10. The cathode volume is Vcat=1.57 cm³.The maximum discharge energy density of the zinc cathode in Sample 10 isWDmax/Vcat=0.62 W-hr/cm³.

Zinc cathode can yield up to 100% utilization of metallic zinc. As shownin FIGS. 2 and 5, the area-specific resistances, ρ^(C) and ρ^(D), of thezinc cells demonstrate relatively flat responses over the DoD range. Thecombination of lower open-circuit potential, and higher maximumpotential translates to a relatively higher maximum charging rate. Insome applications, such as a diesel electric locomotive withregenerative braking, the maximum charging rate can be a significantdeterminant of overall battery performance, where only short bursts ofregenerative energy are available for charging the battery system.

With a zinc-based cathode there is a DoD-dependent open-circuitpotential, as shown in FIGS. 1 and 4. The zinc cathode exhibitsconstantly changing, but predictable, open-circuit voltage potentialversus the depth or state of charge. This can indicate the state ofcharge or the DoD if the associations are provided for the specific cellused. It is therefore possible to predict or determine state of chargewithout cumulative charge tracking and periodic resets.

FIG. 6 shows cell resistance for zinc cell and nickel cell during chargeand discharge cycles. The resistance curves for zinc cell during charge(120) and discharge (130) are relatively flat. With respect to the zinccell, the resistance level of the nickel cell for charge (140) anddischarge (150) are relatively high for entire state of charge. Highcell resistance of nickel cell during charge of the cell is due to aninsulating film formation on cathode. In contrast to the nickel cell,the zinc cell is free of insulating film formation during celloperation.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termsuch as “about” is not to be limited to the precise value specified. Insome instances, the approximating language may correspond to theprecision of an instrument for measuring the value. Similarly, “free”may be used in combination with a term, and may include an insubstantialnumber, or trace amounts, while still being considered free of themodified term. As used herein, the terms “may” and “may be” indicate apossibility of an occurrence within a set of circumstances; a possessionof a specified property, characteristic or function; and/or qualifyanother verb by expressing one or more of an ability, capability, orpossibility associated with the qualified verb. Accordingly, usage of“may” and “may be” indicates that a modified term is apparentlyappropriate, capable, or suitable for an indicated capacity, function,or usage, while taking into account that in some circumstances themodified term may sometimes not be appropriate, capable, or suitable.For example, in some circumstances an event or capacity can be expected,while in other circumstances the event or capacity can not occur—thisdistinction is captured by the terms “may” and “may be”.

The foregoing examples are illustrative of some features of theinvention. The appended claims are intended to claim the invention asbroadly as has been conceived and the examples herein presented areillustrative of selected embodiments from a manifold of all possibleembodiments. Accordingly, it is Applicants' intention that the appendedclaims not limit to the illustrated features of the invention by thechoice of examples utilized. As used in the claims, the word “comprises”and its grammatical variants logically also subtend and include phrasesof varying and differing extent such as for example, but not limitedthereto, “consisting essentially of” and “consisting of.” Wherenecessary, ranges have been supplied, and those ranges are inclusive ofall sub-ranges there between. It is to be expected that variations inthese ranges will suggest themselves to a practitioner having ordinaryskill in the art and, where not already dedicated to the public, theappended claims should cover those variations. Advances in science andtechnology may make equivalents and substitutions possible that are notnow contemplated by reason of the imprecision of language; thesevariations should be covered by the appended claims.

1. A method, comprising: forming brass from zinc powder and copperpowder in the presence of a sodium electrolyte.
 2. The method as definedin claim 1, wherein forming comprises loading an electrochemical cellwith copper and zinc, heating the electrochemical cell, and performingat least one charge/discharge cycle of the electrochemical cell.
 3. Themethod as defined in claim 2, further comprising controlling the zinc tocopper ratio of material within the electrochemical cell so that thematerial is zinc rich in a first location, and is copper rich in asecond location spaced from the first location.
 4. The method as definedin claim 3, wherein the first location is proximate anelectrically-insulating ion-conductive separator.
 5. The method asdefined in claim 3, wherein the second location is proximate anelectrically-conductive current collector.
 6. The method as defined inclaim 3, further comprising reacting the zinc at the first locationprior to reacting the zinc at the second location during the at leastone charge/discharge cycle.
 7. The method as defined in claim 2, whereinheating the electrochemical cell comprises achieving and maintaining atemperature in a range of from about 360 degrees Celsius to about 500degrees Celsius.
 8. The method as defined in claim 2, further comprisinggalvanically protecting the brass by preferentially reacting the zincduring the at least one charge/discharge cycle.
 9. The method as definedin claim 1, wherein forming comprises loading the cell with salts ofzinc and sodium, and performing a discharge cycle.
 10. A method,comprising: converting alpha brass to gamma brass in the presence ofzinc and sodium electrolyte, or salts of zinc and sodium electrolyte.11. The method as defined in claim 10, further comprising controllingthe zinc to copper ratio of material within the electrochemical cell sothat the material is zinc rich in a first location, and is copper richin a second location spaced from the first location.
 12. The method asdefined in claim 11, wherein the first location is proximate to anelectrically-insulating ion-conductive separator.
 13. The method asdefined in claim 11, wherein the second location is proximate to anelectrically-conductive current collector.
 14. The method as defined inclaim 11, further comprising reacting the zinc at the first locationprior to reacting the zinc at the second location during the at leastone charge/discharge cycle.
 15. The method as defined in claim 14,further comprising galvanically protecting the copper by preferentiallyreacting the zinc during the at least one charge/discharge cycle. 16.The method as defined in claim 10, further comprising heating the alphabrass to a temperature in a range of from about 350 degrees Celsius to500 degrees Celsius.
 17. The method as defined in claim 16, furthercomprising heating the alpha brass to a temperature in a range of fromabout 400 degrees Celsius to 420 degrees Celsius.
 18. A method,comprising: determining a state of charge in an energy storage device,comprising: associating ZnCl₂ concentration in the energy storage devicewith open circuit voltage; associating ZnCl₂ concentration voltage withstate of charge; measuring the open circuit voltage; and determining thestate of charge based on the open circuit voltage measurement, and acorresponding ZnCl₂ concentration for the measured open circuit voltage,and the state of charge for the corresponding ZnCl₂ concentration.
 19. Amethod, comprising: operating an energy storage device having anelectrode comprising both an electrochemically active brass supportstructure comprising zinc alloyed with copper, and unalloyed zinc, at anelectrical potential that is greater than an oxidation potential ofcopper during a period where the brass support structure is galvanicallyprotected from degradation.
 20. A method of forming an energy storagedevice, comprising: disposing a separator into a housing to define acathode chamber and a sealed anode chamber; adding sodium chloride, zincand copper into the cathode chamber; and sealing the cathode chamber.21. The method as defined in claim 20, further comprising: supplyingheat to the cathode chamber to form a melt comprising the electrolyte,zinc and copper, and: applying an electric potential to the melt toform, or to modify, a brass support structure.
 22. The method as definedin claim 21, wherein modify the brass support structure comprisesconverting alpha brass to gamma brass.